1. Field of the invention
The present invention relates to a multiple channel magnetic recording and reproducing apparatus for recording video, audio, data, and other data types to magnetic tape and reproducing the recorded data therefrom, and more particularly to a multiple channel magnetic recording and reproducing apparatus for recording and reproducing the oblique track on the tape by means of a pair of helical scanning heads.
2. Description of the Prior Art
While video cassette recorders and other magnetic recording and reproducing apparatuses for recording data to magnetic tape are very practical and are commonly used today, smaller, digital devices are being developed by applying higher density recording technologies. In order to increase the recording density and maintain compatibility with the installed software base, precise tracking of the head with the data track is essential. Existing technologies enable this by recording a control signal on one edge of the tape length using a fixed magnetic head. This recorded control signal is then reproduced using the same fixed magnetic head. Tracking was controlled to maintain a constant phase relationship between the reproduced control signal and the rotational position of the rotary heads. This is the standard method used on a VHS-format VCR.
The drawback to this method is that a dedicated fixed magnetic head must be used for tracking control. Providing this fixed magnetic head poses problems for the design of compact magnetic recording and reproducing apparatuses because of the need to provide a place for the head to be installed and other such problems. This led to the development of a method (ATF) for controlling tracking of the rotary head using only the reproduced output from the moving playback head and not using a fixed head. This ATF method is used, for example, on 8 mm VCRs. In addition to reducing the size and complexity of the recording/reproducing mechanism, ATF is an effective tracking control method with narrow data tracks and for automatic tracking adjustment.
The sector area ATF method used in DAT (digital audio tape recorder) is suited to recording and reproducing digital signals, with which time-base compression and decompression can be used. Unlike analog signals, continuous time base recording/reproduction is not necessary with digital signals. As a result, one track is divided into plural areas, and a tracking pilot signal is recorded in different areas from data (audio or subcode in DAT) area. These tracking pilot signal areas are normally provided in two areas, specifically the first and second halves of the one track. This is to maintain tracking control even if the data is damaged and one of the pilot signals is lost. When a track deviates from the optimum signal pattern, a servo is driven based on the average value of the tracking signal calculated from pilot signals in two locations to reduce the deviation from normal tracking.
The most important feature of the sector area ATF method is that tracking control is possible even during insertion editing. This means that if this sector area ATF is applied to a digital VCR, a VCR with a separate insertion editing function can be achieved. This is possible by dividing and recording the video, audio, index, and other digital signals to separate areas, and recording the tracking pilot signal to a dedicated pilot signal area. The insertion editing function referred to here is a function for separately rewriting the video, audio, index, and other signals (or any subset thereof) to tape.
Systems for recording, in order from the beginning of the tape, the first pilot area, audio area, video area, and second pilot area are currently available. The audio signal rewriting operation is described below.
When the rotary head scans the first and second pilot areas, the rotary head outputs the reproduced signal. The pilot signal detected from this reproduced signal is used for tracking control. When the rotary head scans the audio area, the new audio signal is recorded by the rotary head, overwriting the old audio signal. Continuity between the new track and the previously recorded (not overwritten) track is thus maintained at the end point of the insertion because normal tracking is maintained during the insertion.
Problems arise, however, when this tracking method is used to enable insertion editing in a system using a pair of heads for recording and reproducing. When a pair of heads is used, the output from only one of the heads is usually used for tracking control. The other head is installed in close proximity to the tracking control head with the relative positions thereof precisely maintained, making it possible control the tracking of this second head at the same time. The problems arise when a position shift occurs in the track between the heads. Specifically, while the head of the channel used for tracking passes through the pilot area at the end of the track (second pilot area), the other channel head will still be scanning the data signal area (the video, audio, and index areas) for a certain period. This is true when the leading head is used for tracking control. When the trailing head is used for tracking control, the same situation occurs as the head passes through the pilot area at the beginning of the track (first pilot area). In either case, there will be a period in which the tracking channel head is in the reproducing mode and the other head is simultaneously in the recording mode necessary to overwrite the data signal. Because of the construction of the rotary transformer on the rotary drum on which rotary heads are mounted, it is not possible to set the heads to separate reading and writing modes for each channel. While the current used for recording is on the order of several milliamperes, the current used for reading is on the order of several microamperes. As a result, the playback signal cannot be detected due to recording current crosstalk, and if this playback signal cannot be detected, tracking control is also not possible.
In other words, even though two pilot areas are provided on the track, the playback signal from the one channel cannot be used for tracking control because it cannot be detected. As a result, when the tape is damaged and the pilot signal which can be detected when reproduced is lost, tracking cannot be maintained. In addition, if the signal reproduced from one channel cannot be detected, mistracking will become extremely high when the track deviates from the optimum pattern.